Sources and activities that suggest listing flow alteration as a candidate cause

The natural flow regime of a stream or river may be altered by various human activities, within the channel or the watershed. The more extensive the relevant sources and activities, the more likely increased flow alteration will impair surface waters.

Figure 5. A combined sewer overflow (CSO) discharge point (i.e., a point source discharge) on the Potomac River in Washington D.C. Courtesy of C.R. Ziegler, U.S. EPA.

Point source inputs: Anthropogenic inputs to streams include untreated sewage and stormwaters (Figure 5), wastewater treatment plants, and industrial operations; these inputs can increase flow or provide a source of flow in an otherwise dry channel. Redirecting flow from one watershed to another, or transbasin diversions, also may increase flow in one stream (point source), while decreasing flow in another (water withdrawal; see below).

Water withdrawal: Surface water and groundwater withdrawals can alter flow by reducing water volumes in streams. Withdrawals may return to the surface/groundwater system at a point further downstream, be removed from the watershed through transpiration by crops, lawns or pastures, or be transferred to another watershed altogether (e.g., water transferred to a different watershed for drinking supply).

Figure 6. Aerial images (each approximately 15 km wide) showing landscape features associated with urban, forestry, and agricultural land uses that can affect infiltration and alter overland flow. From left to right, the first image shows the city of San Francisco, CA, and its associated impervious surfaces which reduce infiltration. The second image shows Okanogan National Forest in north-central Washington, with road networks and patches where trees have been removed by logging activity (highlighted by snow cover in the left two-thirds of the image), generally decreasing the retention of storm water in those areas. The image on the right shows the Arkansas River, immediately upstream of its confluence with the Mississippi River; the patchwork of agricultural fields near the river, in various shades of brown and green, can alter flow regimes through altered infiltration rates and irrigation. Courtesy of Google Earth; accessed 12/21/2006.

Figure 7. An engineered open and trapezoidal concrete channel; this structure increases the efficiency with which overland flow travels across the landscape. Courtesy of C.R. Ziegler, U.S. EPA.

Channel alteration: Structural habitat changes include straightening or restructuring natural watercourses. This can involve adding rip-rap to stabilize a stream bank, installing a dam or road crossing, or removing large woody debris. Reduced sinuosity may shorten the distance water travels and increase water velocity. Engineered channels designed to prevent overbank flooding eliminate or reduce floodplain connectivity (some organisms, such as certain species of spawning fish, rely on floodplain habitats, which are sometimes only made available when storm flows rise above channel banks). Channelization also alters channel bathymetry and gradient, thereby reducing structural habitat heterogeneity and riffle/pool variability.

Impoundment: Detention basins, retention basins, and dams (Figures 9 and 10) can affect flow regime in various ways, via actively controlled or passive-release mechanisms. For example, detention basins tend to reduce peak flows during precipitation events or prevent water from reaching a stream during periods of low flow. Similarly, storage dams may reduce or prevent seasonal variation of downstream flow, and decrease the frequency of floodplain inundation. However, hydroelectric dams may increase variance in flow if they are used for peaking power.

Figure 8. A connection between Rock Creek and a road drainage system in Rock Creek National Park, Washington, D.C. Courtesy of C.R. Ziegler, U.S. EPA.

Figure 9. Glines Canyon Dam, located on the Elwha River in Olympic National Park, Washington, has changed the downstream flow regime and converted the upstream ecosystem into a reservoir (Lake Mills). When the photograph was taken in 2005, the U.S. National Park Service was planning to remove the dam. Courtesy of C.R. Ziegler, U.S. EPA.

Figure 10. A small dam and its associated fish ladder in Rock Creek National Park, Washington, D.C. Courtesy of C.R. Ziegler, U.S. EPA.

In addition to observations of sources discussed above, the following site observations may suggest listing flow alteration as a candidate cause. However, note that single, point-in-time observations also may be indicative of natural channel processes. Flow characteristics, structural habitat conditions, and associated biological attributes at a given point in a stream depend on various in-stream and watershed-wide factors upstream, which can change with time. Therefore, the following site observations should be considered in the context of the associated ecosystem and other local factors, such as precipitation and geology.

Scouring and incision: Channel scouring, incision, or downcutting suggest altered flow conditions characterized by increased peak flow magnitude (discharge and velocity) and/or frequency of high flow events. Scouring flows may act to dislodge organisms (i.e., flow as a proximate cause of impairment) and to alter substrate composition or structural habitat (i.e., flow as a step in a causal chain).

Dry stream: A dry stream bed may suggest including flow alteration as a candidate cause. Land cover changes, for example, can eliminate between-storm baseflow. Note, however, that some streams, particularly in the western U.S., are naturally intermittent due to seasonal and regional precipitation patterns. Whether the dry channel is natural or un-natural, flow alteration may be included because aquatic organisms often are sensitive to the timing and duration of zero flow conditions.

Channel features incongruous with observed flow: Observation of a “normal” precipitation event—for example, an event of magnitude that might happen about once per year or once every other year—for which flow levels do not reach bank-full channel features and/or floodplain terraces, may suggest that larger events (altered flow conditions) are dictating channel geometry and evolution.

Discharge data inconsistencies: A plot showing both precipitation and stream discharge (see Ways to Measure tab) may alert causal assessors of certain flow alterations. Zero baseflow between precipitation events may indicate that land cover alterations (e.g., impervious surfaces) have created a flashy hydrologic system, marked by higher peak flows, but reduced baseflow. Conversely, if discharge is relatively constant regardless of precipitation, wastewater dominance or control of flow by a dam might be altering flow characteristics.

Beaver activity: Signs of beaver activity, such as freshly downed trees (Figure 12) may suggest that flow regime is being altered or has already been changed. Unlike sources and observations discussed in the context of causal assessment and CADDIS, beaver dams and related activities are natural phenomena and have many ecological benefits. If flow alteration is under consideration, causal assessors may benefit from acknowledging and distinguishing between natural and anthropogenic processes.

Anecdotal information may strengthen site observation evidence. A classic example is that of the scientist walking up to a dry stream and a local resident curiously joining the scientist streamside; the resident says, "I remember 50 years ago when this creek had water flowing through it everyday of the year, and now it's bone dry unless it rains." Such information would be sufficient to include flow as a candidate cause, but the credibility of anecdotal information must be checked during the causal analysis.

Fish, invertebrates, and aquatic plants vary in their preferences for specific flow characteristics. Here we present examples of biological effects of flow alteration, reported in several synthesis documents linking the disciplines of hydrology and ecology (The Nature Conservancy 2006, Biggs et al. 2005, Bragg et al. 2005, Bunn and Arthington 2002, Poff et al. 1997, Poff and Ward 1989). The general trend among scientists is to connect flow alteration parameters with biological effects. Effects typically are described in the context of species traits, functional adaptations, life history characteristics, or community structure (Roy et al. 2005, Lytle and Poff 2004, Goldstein and Meador 2004). The following examples are categorized in terms of common flow alteration parameters.

Changes in magnitude and duration of low flows: Cessation of flow, extreme low flows, or prolonged duration of low flow conditions can reduce overall habitat availability by decreasing water volume and wetted channel area. This alteration has been linked to reduced total stream productivity, elimination of large fish, changes in taxonomic composition of fish communities, fewer species of migratory fish, fewer fish per unit area, and a greater concentration of some aquatic organisms (potentially benefiting predators). Prolonged duration of low flows tends to favor invertebrate and fish species that prefer standing-water habitats or species classified as generalists. Conversely, extreme low flows that are not unusual for the channel of interest may benefit aquatic systems, by purging invasive or non-native species maladapted to such conditions.

Changes in frequency and magnitude of peak flows: High-flow events can physically remove species from the channel to a downstream location. The literature generally refers to this process as dislodgement, wash-out, scouring, or flushing of organisms. Mobilization of pebbles, sediment, woody debris, and plant material, in addition to movement of water itself, also can dislodge organisms. More frequent high-flow events can decrease species richness by eliminating or reducing populations that do not fare well under high-flow conditions. Invertebrate assemblages consisting of species with long life cycles may shift compositionally to include more species with relatively short life cycles. Conversely, fewer high-flow events and peak flow events that are lower in magnitude may disconnect the channel from its floodplain, reducing access to fish spawning habitat and juvenile fish nursery areas.

Altered seasonality of flows: Many aquatic organisms rely on consistent seasonal flow patterns (e.g., flow increases with spring snow melt) to cue life history events. Altered or reduced seasonality of flows, including changes in the timing of rising flows and flow peaks, may disrupt natural cues for invertebrate life cycles and for migration, spawning, and egg hatching of fish. For example, Peckarsky et al. (2000) discuss the life cycle traits and cues common to the mayfly genus Baetis in connection to flow regime. Specifically, baetid mayflies oviposit on rocks based partly on the timing of rock appearance above the water line, and therefore, seasonal changes in water depth play a role in cueing reproductive processes of baetid mayflies.

Changes in flow variability: The natural variability of a stream's flow regime usually includes peak flow diversity combined with occasional periods of drought. Dams with regulated releases can stabilize water flow, thus reducing variability. This flow stabilization may provide an opportunity for invasive or exotic species to establish and displace native species. Additionally, native specialist organisms, adapted to a particular combination of high and low flows, may be replaced by generalist species, which may not otherwise compete successfully with native species under more natural flow conditions. Decreased variability in flow can reduce fish populations and diversity of the invertebrate community.

Site evidence that supports excluding flow alteration as a candidate cause

There are no site observations that specifically provide evidence of the absence of flow alteration. General reasons for excluding a candidate from the list are described in Step 2 of the Step-by-Step guide and in Tips for Listing Candidate Causes.

We strongly caution against using benchmarks of effects (e.g., water quality criteria) as evidence for excluding flow alteration from your initial list of candidate causes, because different species have different flow requirements and different sites have different naturally occurring levels of flow.